Apparatus for and method of operating an automatic log on/log off circuit in a telephone system by disconnecting a headset

ABSTRACT

Existing automatic log on/log off systems in telephone systems determine whether or not a workstation is occupied, and that calls can be directed to that workstation, by determining whether or not the amplifier that provides the interface between the telephone system and the occupant&#39;s headset is plugged into the workstation. Recent headsets have a connector in the cord between the headset and the amplifier. This enables the occupant to leave the workstation without unplugging the amplifier, which defeats sensing mechanism of the existing automatic log on/log off system. An amplifier according to the invention replaces the existing amplifier and includes a detector that determines whether the headset is connected to the amplifier, and an activator that activates the existing automatic log on/log off system in the telephone system. In the preferred embodiment, the detector senses the current drawn by the electret microphone in the headset, and the activator activates both current-sensing and voltage/resistance-sensing automatic log on/log off systems.

This is a continuation of Application Ser. No. 07/844,739, filed 2 Mar.1992, now U.S. Pat. No. 5,226,077.

BACKGROUND OF THE INVENTION

Telephone headsets are an important element of modern businessequipment. They provide hands-free communication, and improveproductivity, in a variety of applications, such as operator andinformation services.

One of the main applications of telephone headsets is in connection withautomatic in-bound and out-bound telephone systems. Such systemsgenerally include an Automatic Call Distributor (ACD). In a telephonesystem with an ACD, a computer automatically routes telephone calls toworkstations connected to the system in a way that balances the callload equally between the workstations. Each workstation can be occupiedby a telephone representative. For a telephone system with ACD to workproperly, each telephone representative is required to log on to thesystem each time he/she occupies his/her workstation, and to log off thesystem each time he/she leaves his/her workstation. This is necessary sothat the ACD will route calls to all occupied workstations, and will notroute calls to any unoccupied workstations. If a representative leaveshis/her workstation without logging off, the ACD system will continue toroute calls to that workstation. Such calls are not responded to, or aresponse is delayed. A representative failing to log on to the systemincreases the load on the other representatives, and increases the timerequired to respond to incoming calls. The failure of a representativeeither to log on to or log off the system consequently results in areduced quality of service.

The need for a telephone system to determine whether a workstation isoccupied is not restricted to telephone systems with an ACD. Forexample, the sole telephone operator in a small office is also requiredto log off when away from his/her workstation so that incoming calls donot go unanswered.

To improve the quality of service provided by telephone systems in whichthe presence of a representative or operator at a workstation impactsthe quality of service, it is desirable that logging on and logging offbe automated.

A typical workstation includes a telephone headset connected to anamplifier. The amplifier is, in turn, connected to the workstation,which is connected to the telephone system. The amplifier is powered bycurrent drawn from the telephone system. Automated log on/log offsystems are known in which the representative has to unplug theamplifier from the workstation to log off automatically. The automaticlog on/log off system monitors each workstation to determine whether ornot an amplifier is connected to the workstation. The system logs therepresentative off when it determines that the amplifier has beendisconnected from the workstation. Some types of automatic log on/logoff system log the representative back on when it determines that theamplifier has been re-connected to the workstation. Other types requirethe representative to log back in manually.

Such automatic log on/log off systems determine whether the amplifier isconnected to the workstation by monitoring some parameter that dependson whether or the amplifier is connected, such as the current drawn fromthe telephone system by the amplifier, or the resistance between a pairof contacts.

The log on/log off system just described is regarded as automatic, eventhough the representative has to connect or disconnect the amplifier,because operators prefer to continue to wear their headsets when awayfrom their workstations, especially if the time away from theworkstation is short. To be able to leave the workstation while wearingthe headset, the representative has to unplug the amplifier, and has tocarry the amplifier around while away from the workstation.

Recently, headset manufacturers have made it easier for a representativeto leave his/her workstation while wearing his/her headset by providinga connector in the cord between the headset and the amplifier. Therepresentative can then wear the headset while he/she is away from theworkstation and no longer has to carry the amplifier. However,disconnecting the headset using the connector in the cord defeats theknown automatic log in/log out systems, because the representative canleave the workstation without unplugging the amplifier, anddisconnecting the headset using the connector in the cord does notchange any of the parameters monitored by the known automatic log on/logoff systems.

SUMMARY OF THE INVENTION

The invention provides an amplifier for connection to a telephonesystem. The telephone system includes a circuit for monitoring when aperipheral device, such as the amplifier, is connected to it. Themonitoring circuit operates to detect a change in a parameter whichoccurs when the peripheral device is disconnected from the telephonesystem. The amplifier is also connectable to a telephone headset andpasses signals between the headset and the telephone system. Theamplifier comprises a detecting circuit and an activating circuit. Thedetecting circuit detects when the headset is disconnected from theamplifier and generates an output signal when the headset isdisconnected. The activating circuit operates in response to the outputsignal, and changes the parameter in a manner that emulatesdisconnecting the amplifier from the telephone system, even though theamplifier remains connected.

For example, if the monitoring circuit in the telephone system monitorsan additional pair of contacts in the workstation, the activatingcircuit includes an additional pair of contacts. The activating circuitchanges the resistance between the contacts in response to the detectingcircuit's determination of when the headset is disconnected from theamplifier. If the monitoring circuit in the telephone system monitorsthe current drawn by the amplifier, the activating circuit changes thecurrent drawn by the amplifier in response to the detecting circuit'sdetermination of when the headset is disconnected from the amplifier.Normally, the activating circuit reduces the current drawn by theamplifier when the headset is disconnected from the amplifier.

The detecting circuit detects when the headset is connected bydetermining a change that occurs when the headset is connected. Thechange can be in, for example, the voltage or resistance between twoconductors in the headset cord, or the level of a pilot signal imposedon a conductor of the headset cord. In the preferred embodiment, thedetecting circuit detects a change in the current drawn from theamplifier by the headset.

The monitoring circuit in the telephone system may additionally operateto detect a change in the parameter which occurs when the peripheraldevice is reconnected to the telephone system. In this case, thedetecting circuit may additionally detect when the headset isreconnected to the amplifier, and may generate an additional outputsignal in response to the headset being reconnected. The activatingcircuit may then additionally operate in response to the additionaloutput signal to change the parameter in a manner that emulatesreconnecting the amplifier to the telephone system, even though theamplifier has not been disconnected.

The invention also provides a method of operating an automaticmonitoring circuit in a telephone system by disconnecting a headset froman amplifier connected to the telephone system. The automatic monitoringcircuit monitors when a peripheral device, such as the amplifier, isconnected to the telephone system by detecting a change in a parameterwhich occurs when the peripheral device is disconnected from thetelephone system. The method comprising the steps of detecting when theheadset is disconnected from the amplifier, and generating an outputsignal in response the disconnection; and changing the parameter inresponse to the output signal in a manner to emulate disconnecting theamplifier from the telephone system, even though the amplifier remainsconnected.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a telephone system with an automatic logon/log off system that determines whether or not the amplifier isconnected by sensing the current drawn from the telephone system.

FIG. 2 is a block diagram of a telephone system with an automatic logon/log off system that determines whether or not the amplifier isconnected by sensing the resistance or voltage difference between twoadditional contacts.

FIG. 3 is a block diagram of a headset amplifier with automatic logon/log off detection according to the invention connected between aheadset and a telephone system.

FIG. 4 is a schematic diagram of a first variation of the detector partof a headset amplifier with automatic log on/log off detection accordingto the invention in which the detector detects the voltage between twoadditional conductors in the headset cord.

FIG. 5 is a schematic diagram of a second variation of the detector partof a headset amplifier with automatic log on/log off detection accordingto the invention in which the detector detects the voltage between oneadditional conductor and one of the existing conductors in the headsetcord.

FIG. 6 is a block diagram of a third variation of the detector part of aheadset amplifier with automatic log on/log off detection according tothe invention in which the detector detects the level of a pilot signalsuperimposed upon one of the conductors in the headset cord.

FIG. 7 is a block diagram of the activator part of a headset amplifierwith automatic log on/log off detection according to the invention inwhich the activator activates a current sensing type of automatic logon/log off system.

FIG. 8 is a block diagram of the activator part of a headset amplifierwith automatic log on/log off detection according to the invention inwhich the activator activates a current sensing type of automatic logon/log off system with a low current sensing threshold.

FIG. 9 is a block diagram of the activator part of a headset amplifierwith automatic log on/log off detection according to the invention inwhich the activator activates a resistance/voltage sensing type ofautomatic log on/log off system in which the detector detects thecurrent drawn by the headset microphone and the activator can activateboth current sensing and resistance/voltage sensing automatic log on/logoff systems.

FIG. 10 is a schematic diagram of the preferred embodiment of a headsetamplifier with automatic log on/log off detection according to theinvention.

FIG. 11 is a schematic diagram of a headset amplifier with automatic logon/log off detection according to the invention showing how a differentinterconnect can be used to adapt the amplifier for use with a differenttype of telephone system.

DETAILED DESCRIPTION OF THE INVENTION

A typical telephone system with an automatic log on/log off system isshown in FIG. 1. The workstation 1, which is normally one of severalworkstations connected to the telephone system 16, has an amplifier 6 toprovide an interface between the telephone system 16 and the headset 46.The cord 11 connects the amplifier to the telephone system 16. Theamplifier is powered by the telephone system, and draws a current ofseveral milliamps from the telephone system. The cord 11 includes theconductor 21 for DC power and the conductor 31, which provides theground connection. The transmit signal from the amplifier 6 is normallysuperimposed on the conductor 21. Alternatively, an additional conductor(not shown) in the cord 11 may be used for the transmit signal. Finallythe cord 11 includes receive conductors, which have been omitted forclarity. The cord 11 is interrupted by the connector 36, comprising theplug 36A and the socket 36B, which is normally mounted on theworkstation 1. The connector 36 allows the amplifier 6 and the headset46 to be disconnected from the workstation.

The headset 46 includes an earphone (not shown) and a microphone (notshown). Typically, an electret microphone is used, which requires thatthe amplifier 6 supply DC power of a few volts at between 25 and severalhundred microamps to the headset. The cord 51 interconnects the headsetand the amplifier. The cord 51 includes the conductor 56 for DC powerand the conductor 66, which provides the ground connection. The transmitsignal from the microphone in the headset (not shown) is normallysuperimposed on the conductor 56. Alternatively, an additional conductor(not shown) may be used for the transmit signal. Finally, the cord 51includes receive conductors, which have been omitted for clarity. Thecord 51 is interrupted by the connector 71, comprising the plug 71A andthe socket 71B. The connector 71 enables the representative todisconnect the headset from the amplifier when he/she leaves theworkstation 1. Normally, the connector 71 is mounted in the cord 51close to the headset, but it can be an integral part of the headset oran integral part of the amplifier.

Applications handling a large volume of telephone calls, such astelemarketing, bank customer service, or airline reservations, have aplurality of workstations, each with its own telephone connector 36,amplifier 6, headset 46 and necessary interconnecting cords. Theworkstations are connected to the telephone system 16. The telephonesystem may include an Automatic Call Distributor (ACD) for distributingthe calls evenly between occupied workstations, i.e., workstations atwhich a representative is logged on. Smaller telephone systems lack anACD, but the invention can be applied to a telephone system without anACD.

The telephone system 16 includes an automatic log on/log off system 76that automatically logs a workstation off the telephone system when theoccupant leaves the workstation. Some automatic log on/log off systemsautomatically log the workstation back onto the telephone system whenthe occupant returns. Other automatic log on/log off systems require theoccupant to log back in manually.

The cord 11 connects the amplifier 6 to the telephone system 16. Whenthe amplifier 6 is disconnected from the telephone system 16, theautomatic log on/log off system 76 causes the telephone system to takean action that is appropriate in view of the fact that the workstationis no longer occupied. For example, in a small office system with asingle telephone operator, it can cause the telephone system to gotemporarily into its night service mode so that incoming calls do not gounanswered. In a large system with ACD, it prevents the telephone systemfrom directing calls to the unoccupied workstation. When the amplifier 6is re-connected to the telephone system 16, the automatic log on/log offsystem 76 may cause the telephone system to reverse the actionpreviously taken, or the occupant may have to log back in manually.

FIG. 1 shows a telephone system 16 with an automatic log on/log offsystem controlled by the current sensor 86 that detects whether or notthe amplifier 6 is connected to the telephone system by monitoring thecurrent drawn by the amplifier from the telephone system. Current from apower supply 81 in the telephone system is supplied to the conductor 21,and to the amplifier 6, through the current sensor 86. The currentsensor 86 provides an output signal to the automatic log on/log offsystem 76. When the current supplied to the amplifier is greater than athreshold value, typically about one milliamp, the current sensor 86provides an output signal in a first state that indicates that theamplifier is connected to the telephone system. When the currentsupplied to the amplifier is less than the threshold value, the currentsensor provides an output signal in a second state that indicates thatthe amplifier has been disconnected from the telephone system. The stateof the output signal from the current sensor thus informs the automaticlog on/log off system 76 whether or not the amplifier 6 is connected tothe telephone system 16, and the automatic log on/log off system canthus cause the telephone system to take appropriate action.

The connector 71 in the cord 51 between the amplifier 6 and the headset46 enables the occupant to leave the workstation while continuing towear his/her headset without having to disconnect the amplifier from thetelephone system 16. This defeats the automatic log on/log off system.Because the current drawn from the telephone system 16 by the headset 46is small compared with the total current drawn from the telephone systemby the amplifier 6, it is not practical to change the threshold of thecurrent sensor 86 to detect the change in the current drawn by theamplifier resulting from disconnecting the headset.

FIG. 2 shows the telephone system of FIG. 1 fitted with a knownalternative to the current sensor 86 (FIG. 1) for determining whether ornot the amplifier 6 is connected to the telephone system 11. The cord 11is provided with two additional conductors 91 and 96. The conductors areinterconnected on the amplifier side of the connector 36. Inside thetelephone system 16, the additional conductors 91 and 96 are connectedto the sensor 186, which measures a parameter relating to the twoadditional conductors that depends on whether the amplifier 6 isconnected to the telephone system 16. This could be the resistancebetween the conductors, a voltage difference between the conductors, ana.c. signal level, or some other suitable parameter. The sensor 186provides an output signal to the automatic log on/log off system 76.

When, for example, the resistance between the additional conductors 91and 96 is less than a threshold value, the sensor 186 provides an outputsignal in a first state that indicates that the amplifier is connectedto the telephone system. When the resistance between the conductors isgreater than the threshold value, the sensor 186 provides an outputsignal in a second state that indicates that the amplifier has beendisconnected from the telephone system. The state of the output signalof the sensor 186 thus indicates to the automatic log on/log off system76 whether or not the amplifier 6 is connected to the telephone system16, and the automatic log on/log off system can thus cause the telephonesystem to take appropriate action.

As a common alternative to the arrangement shown in FIG. 2, theadditional conductors 91 and 96 can be connected to the contacts of aswitch on the socket 36B. The contacts are actuated by plugging the plug36A into the socket 36B. When the plug 36A (typically a 1/4 inch phonejack) is not plugged into the socket 36B, the contacts are in firstcondition, e.g., open; when the plug 36A is plugged into the socket 36B,the contacts change to a second condition, e.g., closed.

As an alternative to providing two additional conductors 91 and 96, theadditional conductor 96 can be omitted. The sensor 186 would thenmonitor a parameter relating to the single conductor 91 and one of theother conductors 21 and 31 of the cord 11 that depends on whether theamplifier 6 is connected to the telephone system 16. For example, theadditional conductor 91 can be connected to the power/signal conductor21 on the plug 36A of the connector 36, which is connected to theamplifier. The sensor 186 then determines whether or not the voltage onthe additional conductor 91 is substantially the same as the amplifierpower supply voltage.

The connector 71 in the cord 51 between the amplifier 6 and the headset46 enables the occupant of the workstation to leave the workstationwhile continuing to wear his/her headset without having to disconnectthe amplifier 6 from the telephone system 16. This defeats the automaticlog on/log off system because it does not change any parameter relatingto the additional conductors 91 and 96, or relating to the additionalconductor 91 and another conductor in the cord 11.

FIG. 3 shows a block diagram of an amplifier 100 according to theinvention. As in the known amplifiers shown in FIGS. 1 and 2, theamplifier 100 is connected to the headset 146 by the cord 151 thatincludes the connector 171, comprising the plug 171A and the socket171B, and the two conductors 156 and 166 for power/signal and ground,respectively. The amplifier is connected to the telephone system by thecord 111 that includes the connector 136, comprising the plug 136A andthe socket 136B, the two conductors 121 and 131 for power/signal andground, respectively. The cord 111 may also include the additionalconductors 191 and 196. The conductors in the cords 111 and 151 aredirectly or indirectly connected to the amplifier electronics 105, andprovides the necessary interface between the headset 146 and thetelephone system 116. The amplifier electronics 105 are known andconsequently details of them will not be shown.

The amplifier 100 according to the invention additionally includes thedetector 110, to which one or more of the conductors of the cord 151 areconnected, and the activator 115, to which one or more of the conductorsof the cord 111 are connected.

The detector 110 monitors one or more of the conductors of the cord 151to determine whether or not the headset 146 is connected to theamplifier 100, i.e., to determine whether or not the workstation atwhich the amplifier 100 is located is occupied. The detector output 120is connected to the control input 119 of the activator. The detectoroutput 120 preferably provides a signal having one state when theheadset is connected to the amplifier, and a second state when theheadset is not connected to the amplifier.

The activator 115 activates the existing automatic log on/log off systemin the telephone system 116 in response to the detector output 120. Oneor more of the conductors of the cord 111 are connected to theactivator. Depending on the state of the output 120 of the detector 110,the activator makes it appear to the automatic log on/log off system inthe telephone system as if the amplifier is connected to or disconnectedfrom the telephone system. The action of the activator depends on whatparameter the sensor controlling the automatic log on/log off system inthe telephone system monitors to determine whether or not the amplifieris connected.

For example, an activator for use with the telephone system with thecurrent sensing automatic log on/log off system shown in FIG. 1 changesthe current drawn by the amplifier 100 from the telephone system 116 inresponse to the detector 110. When the detector indicates that theheadset 146 is not connected to the amplifier 100, the activator 115reduces the current drawn by the amplifier 100 to less than thethreshold level of the current sensor 86 (FIG. 1) in the telephonesystem 116. When the amplifier draws less current than the thresholdlevel of the current sensor, it appears to the current sensor, and henceto the automatic log on/log off system in the telephone system, as ifthe amplifier 100 has been disconnected from the telephone system.Accordingly, the automatic log on/log off system in the telephone systemlogs off the workstation to which the amplifier is connected.

FIGS. 4 through 6 show various examples of the detector 110. Theactivator 115, the telephone system 116, and the connections betweenthem are omitted from FIGS. 4 through 6. A fourth, preferred, example ofa detector is shown in FIG. 10. FIG. 4 shows a simple form of thedetector 110. The cord 151 is provided with two additional conductors125 and 130 between the amplifier 101 and the connector 171. Theadditional conductors 125 and 130 are connected to two additional pinson the socket 171A. The additional pins on the plug 171B correspondingto the additional pins on the socket 171A are connected together. Theplug 171A is connected to the headset 146. When the headset is connectedto the amplifier, the plug 171B mates with the socket 171A, and theinterconnected additional pins in the plug 171B cause the resistancebetween the additional conductors 125 and 130 to be low. When theheadset is not connected to the amplifier, the plug 171B is not matedwith the socket 171A, and there is no connection between the additionalconductors 125 and 130. Hence, the resistance between the additionalconductors 125 and 130 is high.

The detector 110 can measure the resistance between the additionalconductors 125 and 130 directly using known techniques. In analternative way of measuring the resistance between the additionalconductors 125 and 130, the conductor 125 is connected to the powersupply of the amplifier, i.e., to the conductor 121 (FIG. 3). Theconductor 130 is held at ground potential by the resistor 135, andprovides the detector output 120. When the headset is connected, theconductor 130, and hence the detector output 120, is substantially equalto the amplifier power supply voltage. When the headset is notconnected, the resistor 135 holds the voltage on the conductor 130, andhence the detector output 120, substantially at ground potential.Amplification, buffering, and inversion can be applied to the detectoroutput 120 if necessary.

FIG. 5 shows an alternative simple form of the detector 110. The cord151 is provided with one additional conductor 130 between the amplifier100 and the socket 171A. The conductor 130 is connected to an additionalpin on the socket 171A. An additional pin on the plug 171B, to which theheadset 146 is connected, corresponding to the additional pin on thesocket 171A is connected to one of the other pins on the plug 171B. InFIG. 5, the additional pin on the plug 171B is connected to the pinconnected to the conductor 156, which carries the power supply for theheadset 146. The additional pin in the plug 171B can also be connectedto the pin in the plug connected to the conductor 166, which carries theground connection to the headset if the detection scheme shown in FIG. 5is inverted. The additional pin in the plug 171B could even be connectedto the pin in the plug connected to one of the receive conductors (notshown) if a detection scheme different from that shown in FIG. 5 isused.

When the headset 146 is connected to the amplifier 100, the plug 171B ismated with the socket 171A, which connects the additional conductor 130to the power supply because of the connection between the pin connectedto the conductor 156 (the power supply/signal conductor) and theadditional pin in the plug 171B. When the headset is not connected tothe amplifier, the plug 171B is not mated with the socket 171A, andthere is no connection between the additional conductor 130 and thepower supply, and the additional conductor floats. The resistor 135between the conductor 130 and ground holds the conductor 130substantially at ground potential when the headset is not connected tothe amplifier, and enables the voltage on the conductor 130 to serve asthe detector output 120.

The additional conductors 125 and 130, or the additional conductor 130and one of the conductors 156 or 166, can be connected to the contactsof a switch (not shown) fitted to the socket 171A. The switch isactuated by plugging the plug 171B into the socket.

FIG. 6 shows a form of the detector 110 that does not require additionalconductors to be added to the cord 151. The detector monitors theconductor 156 that carries the power supply/signal to the headset 146.The oscillator 140 imposes a low-level alternating voltage onto theconductor 156 through the impedance 145 (a resistor is shown). The trap150 isolates the alternating voltage from the headset power supply andthe transmit signal input (not shown) of the amplifier electronics 105.The alternating voltage on the conductor 156 is amplified by theamplifier 155, and rectified and smoothed by the rectifier 160. Theresulting DC voltage is fed into the threshold detector 165, whichprovides the detector output 120. A capacitor 170 is connected betweenthe conductor 156 and the ground conductor 166 in the cord 151 somewherebetween the plug 171B, to which the headset 146 is connected, and theheadset 146. FIG. 6 shows the capacitor 170 located in the headset. Itcan also be located in the plug 171B, or be built into the cord 151between the headset 146 and the plug 171B.

When the headset 146 is not connected to the amplifier 100, thealternating voltage at the node 175 is substantially equal to the outputvoltage of the oscillator 140. Circuit parameters are chosen to make theresulting DC voltage at the output of the rectifier 160 greater than thethreshold voltage of the threshold detector 165. At the frequency of thealternating signal, the impedance of the capacitor 170 is comparablewith the resistance of the resistor 145, so that when the headset 146 isconnected to the amplifier 100, the attenuator formed by the capacitor170 and the resistor 145 reduces the level of the alternating voltage atthe node 175. Circuit parameters are chosen to make the resulting DCvoltage at the output of the rectifier 160 less than the thresholdvoltage of the threshold detector 165. Thus, the output of the thresholddetector changes state depending on whether the headset is connected tothe amplifier.

If the frequency of the alternating signal is close to the audiofrequency band, the arrangement shown in FIG. 6 should be modified byreplacing the capacitor 170 by a serial arrangement of an inductor and acapacitor that is resonant at the frequency of the alternating signal.The resonant circuit has a low impedance at the alternating signalfrequency, and thus attenuates the signal level at the node 175, butpresents a relatively high impedance to the audio signal present on theconductor 156.

The detector 110 may monitor the conductors in the cord 151 inadditional ways to determine whether or not the headset 146 is connectedto the amplifier 110.

FIG. 3 shows the activator 115 is connected to at least one of theconductors of the cord 111 between the amplifier 100 and the telephonesystem 116. In response to the detector output 120, the activator 115activates the existing automatic log on/log off system in the telephonesystem 116 by emulating the effect of disconnecting the amplifier fromthe telephone system. The activator 115 includes a controlled switchthat is responsive to the detector output. The controlled switch in theactivator switches the parameter that the sensor in the automatic logon/log off system in the telephone system monitors to determine whetheror not the amplifier is connected to the telephone system.

A version of the amplifier 100 for use with a telephone system having anautomatic log on/log off system having a current sensor that senses thecurrent drawn by the amplifier from the telephone system is shown inFIG. 7. In FIG. 7 the ground conductor 131 is connected directly fromthe telephone system 116 to the amplifier electronics 105. The conductor121 is connected to the power supply input connections 180 and 185 ofthe activator 115 and the detector 110, respectively. The capacitor 126couples the transmit output of the amplifier electronics to the powersupply/signal conductor 121.

The conductor 121 is also connected to one of the switched terminals 104of the controlled switch 190 in the activator 115. The other switchedterminal 109 of the controlled switch is connected to the power supplyinput terminal 114 of the amplifier electronics 105. The detector output120 is connected to the control input terminal 119 of the controlledswitch.

The controlled switch 190 can be a relay, a transistor, an opto-relay orany other suitable switching device that can be controlled by a suitablecontrol signal.

When detector output 120 is in one of its states, the controlled switch190 is in its ON (conducting) state, and when the detector output 120 isin the other of its states, the controlled switch 190 is in its OFF(non-conducting) state. When the controlled switch is in its ON state,the amplifier electronics 105 draw current from the conductor 121 (andhence from the telephone system) through the controlled switch. To thecurrent sensor 86 in the telephone system 116 that monitors the flow ofcurrent through the conductor 121, this appears the same as if theamplifier 100 were connected to the telephone system. When thecontrolled switch is in its OFF state, the amplifier electronics 105draw no current from the conductor 121. To the current sensor 86 in thetelephone system that monitors the flow of current through the conductor121, this appears the same as if the amplifier 100 were disconnectedfrom the telephone system. Thus, by controlling the current drawn fromthe telephone system 116 by the amplifier 100, the controlled switch 190is able to control the automatic log on/log off system in the telephonesystem.

When the controlled switch 190 is in its OFF state, the detector 110 andthe activator 115 continue to draw current from the telephone system116. These circuits must therefore be designed such that together theydraw significantly less current than the threshold current level of thecurrent sensor 86 in the telephone system. The current drawn by thedetector and the activator must be minimized especially when thecontrolled switch is in its OFF state, i.e., when the headset 146 isdisconnected. For example, if a relay is used for the controlled switch190, the normally-open contacts of the relay should be used to providethe switched contacts 104 and 109 so that the relay does not drawcurrent in its OFF state.

As an alternative to switching the power supply to the amplifierelectronics 105, the controlled switch 190 can switch the groundconnection to the amplifier electronics. This enables an NPN transistorto be used for the controlled switch.

If the current sensor 86 controlling the automatic log on/log off systemin the telephone system has a very low threshold, the detector 110 andthe activator 115 should be connected to a power source that is notsensed by the current sensor 86 in the telephone system.

Alternatively, the detector 110, the activator 115, and the amplifierelectronics 105 all can be powered from a power source that is notsensed by the current sensor 86 in the telephone system 116, as shown inFIG. 8. An additional conductor 124 is added to the cord 111. In theamplifier, the conductor 124 is connected to the power supply inputterminals 185, 180, and 114 of the detector, the activator, and theamplifier electronics 105, respectively. The conductor 124 is connecteddirectly to the power supply 81 in the telephone system, bypassing thecurrent sensor 86. Alternatively, the additional conductor 124 can bedispensed with, and the amplifier 100 can be provided with its owninternal DC power supply powered from the a.c. line.

In versions of the amplifier 100 in which the power supply to theamplifier electronics is not switched, a load resistor 129 is connectedbetween the conductor 121, carrying the power supply current that issensed by the current sensor 86, and one of the switched terminals 104of the controlled switch 190. The other switched terminal 109 of thecontrolled switch is connected to the conductor 131 that carries theground connection. The control input terminal 119 of the controlledswitch is connected to the detector output 120. The value of the loadresistor 129 is chosen so that when the controlled switch 190 is in itsON state, the current through the conductor 121, and hence through thecurrent sensor 86, is well above the threshold current of the currentsensor 86. This arrangement has the following advantages: an NPNtransistor with a suitable current rating can be used for the controlledswitch 190; the current drawn from the conductor 121 can be madecompletely independent of the current requirements of the amplifier 100;and the amplifier electronics 105 remain powered when the headset 146 isdisconnected, which avoids unpleasant and potentially harmful transientsin the earphone of the headset when the headset is reconnected.

A version of the activator 115 for use with a telephone system having anautomatic log on/log off system that senses the change in resistancebetween two additional conductors in the cord 111 is shown in FIG. 9. InFIG. 9, the conductors 121, carrying the positive power supply, and 131,carrying the ground connection, are connected from the telephone system116 to the amplifier electronics 105. The additional conductors 191 and196 in the cord 111 are connected to the switched terminals 134 and 139of the controlled switch 195. The control input terminal 119 of thecontrolled switch 195 is connected to the detector output 120.

When the detector output 120 is in one of its states, the controlledswitch 195 is in its ON (conducting) state, and when the detector outputis in the other of its states, the controlled switch 195 is in its OFF(non-conducting) state. When the controlled switch is ON, the conductor191 is connected to the conductor 196 which, to the sensor 102 in thetelephone system 116 appears the same as the amplifier being connectedto the telephone system. When the controlled switch is OFF, theconductor 191 is not connected to the conductor 196 which, to the sensorin the telephone system appears the same as the amplifier not beingconnected to the telephone system. Thus, controlling the resistancebetween the conductors 191 and 196 by the controlled switch 195activates the automatic log on/log off system in the telephone system.

When the telephone system has an automatic log on/log off system thatsenses the resistance or voltage between the contacts of a switchmounted on the socket 136B, the switch being operated by plugging theplug 136A into the socket, the additional conductors 191 and 196 must beconnected either in series or in parallel with the switch, depending onthe operating sense of the switch. Thus, if the switch is open when theplug 136 is plugged in, the additional conductors 191 and 196 must beconnected in parallel with the switch. If the switch is closed when theplug 136 is plugged in, the additional conductors 191 and 196 must beconnected in series with the switch.

The controlled switch 195 can be a relay, a transistor, an opto-relay orany other suitable switching device that can be controlled by a suitablecontrol signal. Preferably, the controlled switch is of the type thathas switched contacts that are isolated from the control terminal sothat the amplifier can be used with telephone systems having any type ofautomatic log on/log off sensor.

FIG. 10 shows the preferred embodiment of the amplifier according to theinvention. The preferred embodiment of the invention is for use with aheadset that includes an electret microphone 103 that draws a small DCcurrent from the amplifier 100 through the power supply/signal conductor156. The detector 110 monitors the current in the conductor 156 anddetects the change in DC current that occurs when the headset isdisconnected from the amplifier, and the microphone no longer drawscurrent. The current in the conductor 156 monitored by the detector 110is about one hundredth of the current in the conductor 121 monitored bythe sensor 86 in the current sensing automatic log on/log off system inthe telephone system shown in FIG. 1.

The headset 146 is connected to the amplifier 100 with the cord 151 thatis interrupted by the plug 171B and the socket 171A that comprise theconnector 171. The cord 151 includes the power supply/signal conductor156 and the ground conductor 166, both of which are connected to theamplifier electronics 105 in the amplifier 100. The cord 151 may bedetachably connected to the amplifier 100 with a suitable connector (notshown) mounted on the amplifier.

Mounted on the amplifier 100 is the socket 108A, which, together withthe plug 108B, forms the connector 108. The power supply/signalconductor 221 is connected between a pin on the socket 108A and thepower supply input terminals 185, 180, and 183 of the detector 100, theactivator 115, and the microphone power supply 123, respectively. Theground conductor 231 is connected between a pin on the socket 108B andthe amplifier electronics 105 and the detector 110. The additionalconductors 291 and 296 are connected between the pins of the socket 108Aand the activator 115. The capacitor 126 couples the transmit output ofthe amplifier electronics 105 to the power supply/signal conductor 221.

The interconnect 113 connects the socket 108A on the amplifier 100 tothe existing socket 136B on the workstation 1. Pins on the socket 136Bare connected to the telephone system 116 by the conductors 321 for thepower supply/signal, 331 for ground, and, optionally, depending on whattype of automatic log on/log off system sensor used by the telephonesystem, the additional conductors 391 and 396.

The interconnect 113 includes the plug 108B, which plugs into the socket108A on the amplifier 100, the cord 118, and the plug 136A, which plugsinto the socket 136B. The cord includes the conductors 121 for the powersupply/signal, 131 for ground, and the additional conductors 191 and196. Choosing the appropriate type of interconnect 113 from a number ofavailable types of interconnect enables the same amplifier 100 to beconnected to a variety of different telephone system workstations. Eachtype of interconnect has the same plug 108B, but, depending on theworkstation to which it is designed to be connected, each type can havea cord 118 with a different number of conductors, a different plug 136A,and the conductors of the cord 118 can be connected to different pins onthe plug 136A.

In the amplifier 100, the amplifier electronics 105 include the powersupply 123, which produces a DC voltage suitable for powering theelectret microphone 103 in the headset 146 from the power supply broughtinto the amplifier from the telephone system 116. The voltage from thetelephone system is in the range of 3 to 12 volts and the voltagerequired for the electret microphone is about 1.2 volts. The DC outputof the power supply 123 is connected through the resistor 128 to theconductor 156, and hence to the microphone 103. When the microphonepicks up speech, it draws a varying current from the power supply 123.The resulting variations in the voltage at the end of the resistor 128remote from the power supply 123 are coupled by the capacitor 133 to amicrophone amplifier (not shown) in the amplifier electronics 105. Inthe amplifier 100 according to the invention, the power supply inputterminal 183 of the power supply 123 is connected directly to theconductor 221, instead of to the power supply connection 114 of theamplifier electronics 105. The power supply 123 must be designed so thatit draws minimal current when the headset is disconnected. This is sothat the amplifier can be used with telephone systems having a currentsensing automatic log on/log off system with a low current sensingthreshold.

The output of the power supply 123 is also connected to the detector110, where is it divided by the potential divider formed by resistors138 and 143. The ratio of the values of resistors 138 and 143 is suchthat the voltage at the junction of the resistors is about 50 millivoltsless than the output voltage of the power supply 123. This voltagedifference may be reduced if a microphone drawing less than about 40microamps is used.

The junction of the resistors 138 and 143 is connected to the invertinginput of the comparator 148. The non-inverting input of the comparator148 is connected to junction of the resistor 128 and the capacitor 133.The output of the comparator 148 provides the detector output 120. Thecomparator 148 is powered by the power supply voltage on the conductor221, through suitable decoupling (not shown), if necessary. A suitableintegrated circuit comparator can be used for the comparator 148.Alternatively, a suitable high-gain integrated circuit operationalamplifier can be used. The comparator should draw a low standing currentbecause the detector 110 draws current when the headset is disconnected.Using a comparator with a low standing current enables the amplifier tobe used with telephone systems having a current sensing automatic login/log out system with a low current sensing threshold. In the preferredembodiment, one-half of a TLC 25L2 dual low-power operational amplifiermade by Texas Instruments is used for the comparator 148.

When the headset 146 is connected to the amplifier 100, and themicrophone 103 draws current from the power supply 123 through theresistor 128 and the conductor 156, the current drawn by the microphonecauses a voltage drop across the resistor 128 of greater than 50 mV.Consequently, the voltage at the junction of the resistor 128 and thecapacitor 133 is more than 50 mV below the output voltage of the powersupply 123. This means that the voltage on the non-inverting input ofthe comparator 148 is lower than the voltage on the inverting input, andthe output of the comparator, i.e., the detector output 120, issubstantially at ground potential.

When the headset is disconnected from the amplifier 100 by parting theconnector 171, the microphone 103 no longer draws current through theresistor 128. As a result, there is substantially no voltage drop acrossthe resistor 128, and the voltage on the non-inverting input of thecomparator 148 is higher than the voltage on the inverting input. Thevoltage on the output of the comparator, i.e., the detector output 120,is substantially equal to the voltage of the positive power supply onthe conductor 221. The detector output 120 thus depends on whether ornot the headset 146 is connected to the amplifier.

In the preferred embodiment of the amplifier according to the inventionshown in FIG. 10, the activator 115 includes two controlled switches.The first controlled switch 190 controls the current that the amplifierdraws from the conductor 221 (and hence from the conductor 321 in thetelephone system), and the second controlled switch 195 switches twoisolated contacts. The control input terminal 119 of the activator 115receives the output 120 from the detector 110. The current drawn fromthe conductor 221 by the amplifier 100 is switched by connecting theconductor 221 and the power supply terminal 114 of the amplifierelectronics 105 to the switched terminal 104 and 109 respectively of thecontrolled switch 190. In the controlled switch 190, the emitter of thePNP transistor 153 is connected to the switched terminal 104 and thecollector of the transistor 153 is connected to the switched terminal109. The base of the transistor 153 is connected to the control inputterminal 119 of the activator 115, and thence to the detector output120, through the resistor 158. A resistor 163 may be connected betweenthe base and the emitter of the transistor 153 if desired. Suitableamplification, buffering, and inversion may be connected between thecontrol terminal input 119 and the transistor 153 if necessary.

When the headset 146 is disconnected from the amplifier 100, the voltageon the detector output 120 is close to the power supply voltage on theconductor 221. No current flows from base of the transistor 153 throughthe resistor 158 into the comparator 148, and the transistor 153 is OFF.Consequently, the amplifier electronics 105 draw no current from theconductor 221, and hence from the telephone system 116. The only currentdrawn from the telephone system 116 is the current drawn by the detector110 and the power supply 123, which is small compared with the currentdrawn by the amplifier electronics 105. Neither the activator shown northe amplifier electronics 105 draws current.

When the headset 146 is connected to the amplifier 100, the voltage onthe detector output 120 is low, and current flows from base of thetransistor 153, through the resistor 158, and into the comparator 148,turning the transistor 153 ON. This connects the amplifier electronics105 to the conductor 221, and allows the amplifier electronics 105 todraw current from the conductor 221, and hence from the telephone system116. The total current drawn from the telephone system 116 is equal tothe current drawn by the detector 110, the activator 115, the powersupply 123 and the amplifier electronics 105.

The activator 115 causes the current drawn from the telephone system 116when the headset is connected to be considerably greater than that drawnwhen the headset is disconnected. The change in current drawn from thetelephone system is sufficiently great to activate the current sensingautomatic log on/log off system in the telephone system 116. Moreover,the change in current drawn from the telephone system along theconductor 321 causes a corresponding change in the current returned tothe telephone system along the ground conductor 331. Thus, the activator115 will also activate an automatic log on/log off system that sensesthe current in the ground conductor 331.

To enable a single amplifier to be used with telephone systems havingboth current sensing and resistance/voltage sensing automatic log in/logout systems, the preferred embodiment of the amplifier includes thesecond controlled switch 195 in the activator 115. The second controlledswitch 195 is preferably an opto-isolator. A relay can also be used forthe controlled switch 195, especially if a relay is used for the firstcontrolled switch 190, since a second pair of switched contacts on thesame relay can provide the second controlled switch 195. Finally, thesecond controlled switch 195 can be provided by some other form ofcontrolled switch.

The switched terminals 134 and 139 of the second controlled switched areconnected by the conductors 291 and 296 respectively to pins on thesocket 108A on the amplifier 100, and thence, via the conductors 191 and196 in the cord 118, and the conductors 396 and 391 to the automatic logon/log off detector (not shown) in the telephone system 116. Theswitched terminals 134 and 139 are connected to the collector andemitter, respectively, of the phototransistor 168 of the opto-coupler.The anode of the photodiode 173 of the optocoupler is connected to thepower supply terminal 180 of the activator; the cathode of thephotodiode 173 is connected through the resistor 178 to the controlinput terminal 119 of the activator, and thence to the detector output120.

When the headset 146 is connected to the amplifier 100, the voltage onthe detector output 120 is low, and current flows through the photodiode173 and the resistor 188 into the comparator 148. The photodiode 178illuminates the phototransistor 168, turning the phototransistor 168 ON.This produces a low impedance between the switched terminals 134 and139, and hence between the conductors 191 and 196. To aresistance/voltage sensing automatic log on/log off system in thetelephone system 116 this low resistance appears the same as if theamplifier were connected to the workstation 1. Accordingly, theautomatic log on/log off system will automatically log the occupant ofthe workstation on.

When the headset 146 is not connected to the amplifier 100, the voltageon the detector output 120 is high, and no current flows through thephotodiode 173 and the resistor 188 into the comparator 148. Thephotodiode 178 does not illuminate the phototransistor 168, and thephototransistor 168 is OFF. This produces a high impedance between theswitched terminals 134 and 139, and hence between the conductors 191 and196. To a resistance/voltage sensing automatic log on/log off system inthe telephone system 116 this appears as if the amplifier weredisconnected from the workstation 1. Accordingly, the automatic logon/log off system will log the occupant of the workstation off.

The ability of the amplifier according to the invention to work withtelephone systems having different types of automatic log on/log offsystems by using a different interconnect 213 is shown in FIG. 11. Thetelephone system 216 is connected to the socket 236B on the workstation1 by a three-wire cord that includes the conductors 321 and 331 forpower/signal and ground, respectively, and the additional conductor 391.The automatic log on/log off system in the telephone system 216 senseswhether the additional conductor 391 is at the same potential as thepositive supply conductor 321.

The interconnect 213 enables the amplifier 100, which is the same as theamplifier shown in FIG. 10, to operate the automatic log on/log offsystem of the telephone system 216, which is different from theautomatic log on/log off system in the telephone system 116 shown inFIG. 10. The interconnect 213 has the plug 108B that plugs into thesocket 108A on the amplifier 100, and the plug 236A that is of thecorrect type to mate with the socket 236B on the workstation 1. The cord118 has the conductors 121 and 131 for power supply/signal and ground,respectively, and the additional conductors 191 and 196, as in FIG. 10.The additional conductors 191 and 196 are connected through theconnector 108 to the switched contacts 134 and 139, respectively, of thesecond controlled switch 195 in the activator 115.

So that the second controlled switch 195 can change the voltage on theadditional conductor 391, the interconnect 218 includes a link 193between the conductor 121 carrying the positive supply voltage and theadditional conductor 196. The link is shown mounted on the plug 236A,but it could alternatively be mounted on the plug 108B, or in the cord118 between the plug 108B and the plug 136A.

When the second controlled switch 195 is ON, the additional conductor191 is connected to the additional conductor 196, which is at thepotential of the positive supply voltage. When the second controlledswitch is OFF, the additional conductor 191 is at a voltage differentfrom the voltage of the positive supply. The second controlled switch195, together with the appropriate interconnect 218, controls thepotential on the additional conductor 191 in such a way as to activatethe voltage sensing automatic log on/log off system of the telephonesystem 216.

We claim:
 1. An amplifier for connection to a telephone system, thetelephone system including a means for monitoring when a peripheraldevice is connected thereto, the monitoring means operating to detect achange in a parameter which occurs when the peripheral device isdisconnected from the telephone system, the amplifier also beingconnectable to a telephone headset, and passing signals between thetelephone headset and the telephone system, the amplifiercomprising:detecting means for detecting when the headset isdisconnected from the amplifier and for generating an output signal inresponse thereto; and activating means, operating in response to theoutput signal, for changing the parameter in a manner that emulates thedisconnection of a peripheral device from the telephone system, eventhough the amplifier remains connected.
 2. The amplifier of claim 1,wherein:the parameter is a current drawn from the telephone system, theamplifier further comprises a current drawing means for drawing currentfrom the telephone system, and the activating means changes the currentdrawn from the telephone system by the current drawing means.
 3. Theamplifier of claim 2, wherein the current drawn from the telephonesystem by the current drawing means when the headset is connected to theamplifier is greater than when the headset is not connected to theamplifier.
 4. The amplifier of claim 2, wherein the activating meansincludes a controlled switch having:a first switched contact connectedto a conductor through which the amplifier draws current, a secondswitched contact connected to the current drawing means, and a controlinput terminal receiving the detector output signal.
 5. The amplifier ofclaim 2, wherein the current drawing means is a resistor.
 6. Theamplifier of claim 1, wherein the detecting means is responsive to thecurrent drawn from the amplifier by the headset.
 7. The amplifier ofclaim 6, wherein the detecting means includes:means for generating avoltage substantially proportional to the current drawn from theamplifier by the headset, and comparator means for comparing the voltagewith a reference voltage and for providing the output signal.
 8. Theamplifier of claim 1, wherein:the parameter is a resistance between afirst conductor and a second conductor, the activating means includes acontrolled switch having a first switched contact coupled to the firstconductor, and a second switched contact coupled to the secondconductor, the first switch contact and the second contact having aresistance between them, and the resistance between the first switchedcontact and the second switched contact changes in response to theoutput signal.
 9. The amplifier of claim 1, wherein:a cord, including aconnector and plural conductors, connects the headset to the amplifier;and the detecting means includes a means for determining when adetection parameter changes as a result of the headset beingdisconnected from the amplifier, the detection parameter being chosenfrom a group including:a current through one of the plural conductors, aresistance between two of the plural conductors, and a voltage on one ofthe plural conductors.
 10. The amplifier of claim 9, wherein:the cordincludes two conductors having a resistance between them, the resistancebetween the two conductors changing when the headset is disconnectedfrom the amplifier, and the determining means determines when theresistance between the two conductors changes as a result of the headsetbeing disconnected from the amplifier.
 11. The amplifier of claim 10,wherein the determining means applies a voltage to one of theconductors, and the voltage on the other of the conductors provides theoutput signal.
 12. The amplifier of claim 9, wherein:the cord includesconductors for power supply, signal, and ground, and an additionalconductor, and the determining means provides the voltage on theadditional conductor as the output signal.
 13. The amplifier of claim 9,wherein:the cord includes a pair of connections for power supply andground, and the determining means includes:means for applying a pilotsignal having an amplitude to one of the conductors, and means formeasuring the amplitude of the pilot signal on the one of the conductorsand for providing the output signal in response to the measuredamplitude of the pilot signal.
 14. The amplifier of claim 2, wherein themonitoring means of the telephone system additionally operates to detecta change in the parameter which occurs when the peripheral device isreconnected to the telephone system, and wherein:the detecting means isadditionally for detecting when the headset is reconnected to theamplifier, and for generating an additional output signal in responsethereto; and the activating means additionally operates in response tothe additional output signal to change the parameter in a manner thatemulates the reconnection of a peripheral device to the telephonesystem, even though the amplifier has not been disconnected.
 15. Theapparatus of claim 2, wherein the monitoring means of the telephonesystem additionally operates to detect a change in the parameter whichoccurs when the peripheral device is reconnected to the telephonesystem, and wherein:the detecting means generates the output signal in afirst state when the headset is disconnected from the amplifier; theactivating means operates in response to the first state of the outputsignal to change the parameter in a manner that emulates thedisconnection of a peripheral device from the telephone system, eventhough the amplifier remains connected; the detecting means isadditionally for detecting when the headset is reconnected to theamplifier, and for generating the output signal in a second state inresponse thereto; and the activating means additionally operates inresponse to the second state of the output signal to change theparameter in a manner that emulates the reconnection of a peripheraldevice to the telephone system, even though the amplifier has not beendisconnected.
 16. The amplifier of claim 8, wherein the monitoring meansof the telephone system additionally operates to detect a change in theparameter which occurs when the peripheral device is reconnected to thetelephone system, and wherein:the detecting means is additionally fordetecting when the headset is reconnected to the amplifier, and forgenerating an additional output signal in response thereto; and theactivating means additionally operates in response to the additionaloutput signal to change the parameter in a manner that emulates thereconnection of a peripheral device to the telephone system, even thoughthe amplifier has not been disconnected.
 17. The apparatus of claim 8,wherein the monitoring means of the telephone system additionallyoperates to detect a change in the parameter which occurs when theperipheral device is reconnected to the telephone system, andwherein:the detecting means generates the output signal in a first statewhen the headset is disconnected from the amplifier; the activatingmeans operates in response to the first state of the output signal tochange the parameter in a manner that emulates the disconnection of aperipheral device from the telephone system, even though the amplifierremains connected; the detecting means is additionally for detectingwhen the headset is reconnected to the amplifier, and for generating theoutput signal in a second state in response thereto; and the activatingmeans additionally operates in response to the second state of theoutput signal to change the parameter in a manner that emulates thereconnection of a peripheral device to the telephone system, even thoughthe amplifier has not been disconnected.
 18. The amplifier of claim 1,wherein the monitoring means of the telephone system additionallyoperates to detect a change in the parameter which occurs when theperipheral device is reconnected to the telephone system, andwherein:the detecting means is additionally for detecting when theheadset is reconnected to the amplifier, and for generating anadditional output signal in response thereto; and the activating meansadditionally operates in response to the additional output signal tochange the parameter in a manner that emulates the reconnection of aperipheral device to the telephone system, even though the amplifier hasnot been disconnected.
 19. The apparatus of claim 1, wherein themonitoring means of the telephone system additionally operates to detecta change in the parameter which occurs when the peripheral device isreconnected to the telephone system, and wherein:the detecting meansgenerates the output signal in a first state when the headset isdisconnected from the amplifier; the activating means operates inresponse to the first state of the output signal to change the parameterin a manner that emulates the disconnection of a peripheral device fromthe telephone system, even though the amplifier remains connected; thedetecting means is additionally for detecting when the headset isreconnected to the amplifier, and for generating the output signal in asecond state in response thereto; and the activating means additionallyoperates in response to the second state of the output signal to changethe parameter in a manner that emulates the reconnection of a peripheraldevice to the telephone system, even though the amplifier has not beendisconnected.
 20. The amplifier of claim 1, wherein the parameter iscurrent.
 21. The amplifier of claim 1, wherein the parameter isresistance.
 22. A method of operating an automatic monitoring circuit ina telephone system by disconnecting a headset from an amplifierconnected to the telephone system, the automatic monitoring circuitmonitoring when a peripheral device is connected to the telephone systemby detecting a change in a parameter which occurs when the peripheraldevice is disconnected from the telephone system, the method comprisingthe steps of:detecting when the headset is disconnected from theamplifier, and generating an output signal in response thereto; andchanging the parameter in response to the output signal in a manner toemulate the disconnection of a peripheral device from the telephonesystem, even though the amplifier remains connected.
 23. The method ofclaim 22, wherein the parameter is current.
 24. The method of claim 22,wherein the parameter is resistance.
 25. The method of claim 22,wherein:the parameter is a current drawn from the telephone system, theamplifier draws current from the telephone system, and in the step ofchanging the parameter in response to the output signal, the currentdrawn from the telephone system by the amplifier is changed in responseto the output signal.
 26. The method of claim 25, wherein, in the stepof changing the parameter in response to the output signal, the currentdrawn from the telephone system by the amplifier when the headset isconnected to the amplifier is greater than the current drawn from thetelephone system when the headset is not connected to the amplifier. 27.The method of claim 25, wherein the automatic log on/log off system inthe telephone system additionally detects a change in the parameterwhich occurs when the peripheral device is reconnected to the telephonesystem, and wherein the method additionally includes the stepsof:detecting when the headset is reconnected to the amplifier, andgenerating an additional output signal in response thereto; andchanging, in response to the additional output signal, the parameter ina manner that emulates the reconnection of a peripheral device to thetelephone system, even though the amplifier has not been disconnected.28. The method of claim 22, wherein:the parameter is a resistancebetween a first conductor and a second conductor, the first and secondconductors being connected to the telephone system, and in the step ofchanging the parameter in response to the output signal, the resistancebetween the first conductor and the second conductor is changed inresponse to the output signal.
 29. The method of claim 28, wherein:acord including plural conductors connects the headset to the amplifier,and the step of detecting when the headset is disconnected from theamplifier includes the step of determining when a detection parameterchanges as a result of the headset being disconnected from theamplifier, the detection being chosen from a group including:a currentthrough one of the plural conductors, a resistance between two of theplural conductors, and a voltage on one of the plural conductors. 30.The method of claim 29, wherein the automatic log on/log off system inthe telephone system additionally detects a change in the parameterwhich occurs when the peripheral device is reconnected to the telephonesystem, and wherein the method additionally includes the stepsof:detecting when the headset is reconnected to the amplifier, andgenerating an additional output signal in response thereto; andchanging, in response to the additional output signal, the parameter ina manner that emulates the reconnection of a peripheral device to thetelephone system, even though the amplifier has not been disconnected.31. The method of claim 22, wherein the automatic log on/log off systemin the telephone system additionally detects a change in the parameterwhich occurs when the peripheral device is reconnected to the telephonesystem, and wherein the method additionally includes the stepsof:detecting when the headset is reconnected to the amplifier, andgenerating an additional output signal in response thereto; andchanging, in response to the additional output signal, the parameter ina manner that emulates the reconnection of a peripheral device to thetelephone system, even though the amplifier has not been disconnected.32. A method of operating an automatic monitoring system in a telephonesystem by disconnecting a headset from an amplifier connected to thetelephone system, the amplifier being connected to the telephone systemby a connection including a plug and socket coupling, the automaticmonitoring system monitoring when the amplifier is physicallydisconnected from the telephone system by detecting a change in aparameter which occurs when the plug and socket coupling is physicallydisconnected, the method comprising the steps of:disconnecting theheadset from the amplifier while leaving the plug and socket couplingbetween the amplifier and the telephone system physically connected; andchanging, in response to the disconnecting step, the parameter in amanner to emulate disconnecting the plug and socket coupling, eventhough the plug and socket coupling remains physically connected. 33.The method of claim 32, wherein:the method additionally comprises thestep of generating a control signal in response to the disconnectingstep; and in the changing step, the parameter is changed in response tothe control signal.
 34. The method of claim 32, wherein:the amplifierdraws current from the telephone system; the parameter is the currentdrawn by the amplifier from the telephone system; and in the changingstep, the current drawn by the amplifier is reduced in response to thedisconnecting step to emulate disconnecting the plug and socketcoupling.
 35. The method of claim 32, wherein:two conductors interruptedby the plug and socket coupling connect the amplifier to the telephonesystem; the parameter is a resistance between the two conductors; and inthe changing step, the resistance between the conductors is increased inresponse to the disconnecting step to emulate disconnecting the plug andsocket coupling.
 36. The method of claim 32, wherein the parameter iscurrent.
 37. The method of claim 32, wherein the parameter isresistance.